PERMIAN TO JURASSIC REDBEDS OF THE WILLISTON BASIN

C. G. Carlson

North Dakota Industrial Commission Oil and Gas Division

MISCELLANEOUS SERIES 78 NORTH DAKOTA GEOLOGICAL SURVEY John P. Bluem1e, State Geologist 1993

INDUSTRIAL COMMISSION OF NORTH DAKOTA

Eclward T. SclWer Heidi Heitkamp Sarah Vogel GOVERNOR ATTORNEY GENERAL COMMISSIONER OF AGRICULTURE

NORTH DAKOTA GEOLOGICAL SURVEY John P. Bluemle, State Geologist

William A. McClellan, Asst. State Geologist

SURVEY STAFF

Richard A. Baker, Drafting Tedmicilm Robert F. Biek, Geologist Randolph B. Burke, CArbonate Geologist Paul E. Diehl, Geologist LaRae L Fey, Infrn'm4tion Processor Operator Phillip L Greer, Geologist Thomas J. Heck,. Geologist Tracy A. Henman. Infot7/Ul.tion Proassor Operator Darby A. Henke, Infot7lUl.tUm Processor Operator John W. Hoganson, Paleontologist Kent E. Hollands, Core Library Tedmtdan Karen M. Gutenkunst, Business MlUUlger Julie A. LeFever, Geologist/Core Library Director Jim S. Lindholm, Data Processing Coordinator Mark R. Luther, Geologist/GIS Manager Eula M. Mailloux, Clerk Annette M. MAter!, Receptionist Sharon M. Murfield-Tyler, GIS Analyst Edward C. Murphy, Environmental Geologist Russell D. Prange, Lab Tedrnicilltl Evie A. Roberson, Administrative Offiar Dean J. Tyler, GIS AtlIllyst

NORTH DAKOTA GEOLOGICAL SURVEY 1993 600 EAST BOULEVARD, BISMARCK ND 58505~

Mountain Association of Geologists, Denver, Colorado, p. 166-176.

Nordquist, J. W., 1955, Pre-Rierdon Jurassic Stratigraphy in Northern Montana and Williston Basin: Billings Geological Society, Sixth Annual Field Conference Guidebook, p. 96-106.

Peterson, J. A., 1957, Marine Jurassic of Northern Rocky Mountains and Williston Basin: American Association ofPetroleum Geolo­gists BuUelin, v. 41, p. 399-440.

Peterson, J.A., 1972, Jurassic System: in Geologic Atlas of the Rocky MounlJlin Region; edited by Mallory, W. W.; Rocky Moun­tain Association of Geologists, Denver, Colorado, p. 177-189.

Rascoe, Bailey, Jr., and Baars, D. L., 1972, Permian System: in Geologic Atlas of the Rocky Mountain Region; edited by Mallory, W. W., Rocky Mountain Associa­

tion of Geologists, Denver, Colorado, p. 177-189.

Seager, O. A., el al" 1941: American Association Petroleum Geologists BuUetin, v. 26, p. 1415, 1421.

Slott, D. F., 1955, Jurassic Stratigraphy of Manitoba: Manitoba Mines Branch Publi­cation 54-2, 78 p.

Vail, P. R., Mitchum, R. M., and Thompson, S. lII, 1987, seismic Stratigraphy and Global Changes ofSea Level, Part 4; Global Cycles of Relative Change of Sea Level: American Association Petroleum Geologists Treatiseof Petroleum Geology Reprint Series No. I, p. 313-327.

Zieglar, D. L., 1956, Pre-Piper post-Minnekahta Redbeds in the Williston Basin: in First Williston Basin Symposium, North Dakota and Saskatchewan Geological Societies, p. 170-178.

21

CONTENTS

ILLUSTRATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. iv

ABSTRACT 1

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 1

PREVIOUS WORK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 1 South Dakota 1 Manitoba. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 1 Saskatchewan 2 North Dakota 2

NEW INTERPRETATIONS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 3

STRATIGRAPHIC REVIEW. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 5 Permian. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 5 Permo-Triassic 5 Jurassic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 5 Unconformities 13

CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 18

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 20

III

ILLUSTRATIONS

Figure Page 1. Typical log characteristics and stratigraphic nomenclature of redbed strata as

defined by Zieg1ar (1956) and Dow, 1967 . . . . . . . . . . . . . . . . . . . . . . .. 3 2. Typical log characteristics and revised stratigraphic nomenclature for Jurassic

to Permian redbed strata 4 3. Isopach map of the Opeche Salt 6 4. Isopach map of the Opeche Formation 7 5. Isopach map of Minnekahta Formation 8 6. Isopach map of Belfield Member of the Spearfish Formation . . . . . . . . . . . . .. 9 7. Isopach map of "G" Salt " 10 8. Isopach map of "P" Salt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 11 9. Isopach map of Pine Member of the Spearfish Formation . . . . . . . . . . . . . . .. 12 10. Isopach map of Reirdon Formation and Bowes through Kline Members

of the Piper Formation 14 11. Log of Phillips Petroleum Corp. - O. Saude No. 1 well (NW SE 19-158-74)

with revised definition of Saude Member of Piper Formation . . . . . . . . . . .. 15 12. Log of Amerada Petroleum Corp. - Pederson, Cater No.1 well

(NE SW 21-158-95) showing revised definition of Saude Member of Piper Formation 15

13. Isopach map of Saude Member, Piper Formation " 16 14. Isopach map of Dunham Salt, Piper Formation 17 15. Log characteristics of Jurassic and Mississippian strata in the Herman

Hanson - Welder No. I well (NE NW 20-133-72) in Logan County. . . . . . .. 18

Plate (2 total in separate packet) 1. Cross Section A-A'; a north-south section from Williams to

Slope Counties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . envelope Cross Section B-B'; a northwest-southeast section from McKenzie to

Grant Counties Cross Section C-C'; east-west section from Williams to

Renville Counties 2. Cross Section D-D'; east-west section from Renville to

Bottineau Counties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . envelope Cross Section E-E'; northwest-southeast section from Ward to

Wells County

iv

ABSTRACT

Redbeds of Permian to Jurassic age are present in outcrops in the Black Hills of South Dakota. Redbeds. which have been referred to as Jurassic in age, are present in outcrops in southern Manitoba. In the subsuiface. equivalent strata are referred to as the Spearfish Formation of Triassic or Permo-Triassic age in Nonh Dakota and the lower Watrous or Amaranth Formations of Jura­Triassic or Jurassic age in Saskatchewan and Manitoba. Paleontological data are lackingfor age resolution. A careful examination of physical evidence from the subsuiface indicates that previous­ly unrecognized regional unconfonnities occur within the clastic redbeds. Areal distribution patterns ofthese lithogenetic unitsfavorthe Jurassic age assignmentfor a pan ofthe Lower Watrous and Amaranth Formations.

The Spearfish Formation, as originally de­fined. includes redbeds, which have been correlated with stratafrom Permian to Jurassic age in eastern "yoming. although these beds were originally generally assigned a Triassic age. The Spearfish Formation will retain its greatest utility in the subsuiface if it is restricted to strata of Permian to Triassic age. Redtifinition ofthe Belfield and Pine units would avoid the introduction of new names and, by applying them to the two lithogenetic units within the Spearfish. would more accurately portray the depositional history of these strata. Similarly a redefinition of the Saude. closely resembling the original definition. would preserve the utility ofthat term.

INTRODUCTION

Since the eatliest days ofexploration in the Williston Basin the age and correlation of redbeds has been controversial. Zieglar (1956. p. 170) noted at the First Williston Basin Symposium that "on the basis of stratigrapbic position and gross lithology the pte-Piper, post-Minnekahta redbeds in the subsurface ofthe Williston Basin are considered by many American geologists to be correlative with the Spearfisb Formation ofPermo-Triassic age. On the other hand, Canadian workers believe that the pre-Piper redbeds in Manitoba and Saskatcbewan are lower middle or lower Jurassic in age." The strata referred to as Lower Amaranth in Manitoba

or Lower Watrous in Saskatchewan are physically equivalent to strata referred to as Saude or Spearfisb in North Dakota. No paleontological evidence has been found to reconcile tbe differing age interpreta­tions, so conclusive resolution ofthis question is not yet possible. However, a careful examination of physical evidence provided by Dual Laterologs suggests a most likely solution.

Nearly all exploration wells penetrate the complete Mesozoic section so wells with Dual Laterologs were randomly selected at one or two per township in areas of active exploration. All available logs were used in less active areas. This density is sufficient to provide a regional pattern for most of the strata. The halites have erratic patterns in some areas due to solution so details of presence or absence of halite cannot be shown in all areas on tbese maps.

PREVIOUS WORK

South Dakota

The term Spearfish Formation was introduced by Darton (1899. p. 387) to refer to redbeds present between lbe Minnekahta Limestone (Permian) and the Sundance Formation (Jurassic) in the northern Black Hills, South Dakota. The name is derived from tbe town of Spearfisb, but no type section was designated. Lacking paleontological evidence a Triassic (?) age assignment was given on the basis of stratigraphic position. A portion of the strata referred to as Spearfisb in the outcrops of the Black Hills has been correlated with Gypsum Spring (Jurassic) strata ofWyoming (Imlay, 1947, p. 237). The Minnekahta Limestone and overlying redbeds of tbe Spearfisb Formation of South Dakota have been shown as correlative with the Goose Egg Formation of Permo-Triassic age of Wyoming (Rascoe, 1972, p. 160).

Manitoba

Seager and others (1942, p. 1415) introduced tbe term Amaranth Formation for redbeds and evaporites which he referred to as Devonian. Stott (1955, p. 7) defined the term Amaranth Formation as redbeds, evaporites, and carbonates of Jurassic or older age overlying Paleozoic strata in Manitoba. Stott divided tbe Amaranth into an upper carbonate

I

and evaporite member of Jurassic age and a lower, unfossiliferous redbed member, which was also assigned a Jurassic age. The Lower Amaranth was further subdivided (Barchyn, 1982, p. 5) into an upper shaIy unit and a lower sandy unit. He agreed with the Jurassic age assignment of the entire Ama­ranth based on a conformable relationsbip of the Upper and Lower Members of the Amaranth Formation. Barcbyn (1982, p. 107) suggested variable depositional conditions for the lower sandy sequence whereas the consistency oflog characteris­tics of the upper shaly sequence was regarded as evidence for more stable conditions in a marine­associated environment. Husain (1991, p. 152) referred to the Lower Amaranth as Jura-Triassic while noting a lack of paleontological evidence for an age assignment.

Saskatchewan

Milner and Thomas (1954, p. 255) introduced the term Watrous Formation to refer to Jurassic redbeds together with carbonates and evaporites overlying Paleozoic strata in Saskatche­wan (= Amaranth of Manitoba). Francis (1957, p. 376) referred to the lower portion of Watrous strata as the Jura-Triassic redbeds while noting that the contact with the overlying non-clastics member of the Watrous was gradational. Carlson (1968, p. 1978) referred to the Lower Watrous as "equivalent to the Triassic Spearfish southward where the Jurassic-Triassic boundary is undisputed. Because it has been established that the Spearfish is of Late Permian and Early Triassic age ... " (Presumably referring to the South Dakota-Wyoming correla­tions). Christopher (1982, p. 85-87) reviewed the age questions and equated the Lower Watrous of Saskatchewan to the Upper Spearfish floored by the Pine SaIt in North Dakota (Saude ofcurrent usage). He noted that this interpretation suggests an early Triassic age for the lower Watrous and implies an unconformity and a hiatus spanning Middle Triassic and Early Jurassic between the Lower and Upper Watrous. However, on his stratigraphic cross section (1982, fig. 5, p. 87) Christopher places an unconformity at the base of the Watrous, but does not separate the Lower and Upper Watrous with an unconformity. Kreis (1991, p. 167) considered the Lower Watrous as largely Early Triassic based on stratigraphic considerations "with an undefined portion of the uppermost part possibly being Early to Middle Jurassic."

North Dakota

Laird (1951) referred to the redbeds and evaporites overlying the Permian Minnekahta Limestone as the Spearfish Formation of Triassic age. Zieglar (1956, p. 177) proposed a revision, restricting Spearfish to a portion of those strata and introduced the terms PineSalt, SaudeFormationand Dunham Salt (fig. 1) for the rest of these strata. He recognized a pre-Jurassic regional unconformity, which he considered as coincident with the base of the Pine Salt.

Dow (1967, p. 14) introduced the currently accepted nomenclature for these redbeds and evaporites in North Dakota (fig. 1). He recognized the Dunham Salt as a facies of the Poe evaporite member of the Piper Formation so he transferred the Dunham Salt to the Jurassic Piper Formation. He redefined the Saude to include the "G marker bed" and the clastics between the "G marker bed" and the main halite bed, which he then referred to as the Pine Salt. He introduced the term Belfield Member for the clastics between the Minnekahta Limestone and Pine Salt. He used the terms Belfield, Pine, and Saude as members of the Spearfish Formation. He assigned a Permo-Jurassic age to these strata as he did not recognize any unconformity within the redbeds and evaporites. He used Saude and Spearfish synonomously in the area north and east of the limit of Pine Salt.

Rascoe (1972, p. 148) accepted a disconformity at the base of the Pine Salt, but considered it to be an intra-Guadalupe discon­formity. He placed the pre-Jurassic unconformity at the base of the Poe or Dunham evaporites.

The Williston Basin nomenclature cbart (Bluemle, et aI., 1981) places the Triassic-Permian boundary at the top of the Pine Salt with a conform­able relationship between the Spearfish and Minnekahta Formations. The North Dakota Stratigraphic Column (Bluemle, et aI., 1986) has the Saude, Pine, and Belfield as members of the Spearfish Formation, and assigns a Permo-Triassic age to these strata. They recognize a pre-Jurassic unconformity, which they place at the base of the Dunham or Poe Members of the Jurassic Piper Formation.

2

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Figure 1. Typical log characteristics and stratigraphic nomenclature of redbed strata.

NEW INTERPRETATIONS conformities present within the strata currently re­ferred to as the Spearfish Formation in North

Three episodes of halite deposition have Dakota. The unconformities, rather than being at occurred in the Williston Basin from the Permian the base of the halites, are within the clastics through Jurassic time span. The halites and the currently referred to as the Belfield and Saude Minnekahta Limestone are distinctive lithologies that Members of the Spearfish Formation. Recognition have been used to divide these strata into strati­ of these unconformities shows that the Dunham and graphic units. These recognized subdivisions have "P" (Pine) salts, like the Opeche Salt, are part of provided useful mapping units where the halites are depositional episodes which begin and end with present. However, these units are not lithogenetic clastics. The areal extent of the "G Salt" and the units, their use tends to mask the geologic history, presence of clastics between the "G Salt" and the and the clastics of the Belfield and Saude units have underlying halite clearly indicate that they are part been combined as Saude in areas to the north and of the same depositional episode. Referring to the east of the limit of the halite. main halite bed as the "P Salt" eliminates duplica­

tion of the term Pine. Careful examination of Dual Laterologs

indicates that previous interpretations have either I would propose that the terms Saude, Pine, misplaced (Zieglar, 1956; Rascoe, 1972; Bluemle, and Belfield should be redefined to reflect litho­etal., 1981,1986), or ignored (Dow, 1967), un- genetic units. The Saude and Belfield should be

3

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restricted to portions of the strata currently referred of the Spearfish Formation of Permo-Triassic age. to these units with the remaining portions added to I would transfer the Saude as a member to the Piper the Pine. I would use Belfield and Pine as members Formation of Jurassic age (fig. 2).

4

STRATIGRAPIDC REVIEW

Permian

Deposition of the Opeche Formation began with red clastics followed by halite in the central basin area and then red clastics before deposition of the Minnekabta Limestone. Current extent of the halite (fig. 3) indicates depositional thicknesses have been preserved except for the southeast margin through Stark and Dunn Counties where rapid thinning from 100 feet to absent indicates a solutional margin. Depositional thicknesses of the Opeche Formation are preserved where the Minnekabta Formation is present (fig. 4). Similar­Iy, depositional thicknesses of the Minnekabta Formation have been preserved where the Belfield Member of the Spearfish Formation is present (fig. 5).

Two distinct laterolog characteristics (see cross sections A-A' and B-B', plates I and 2) are present within the clastics between the Minnekabta Limestone and the halite referred to as the Pine Salt. I believe this marks an unconformity and I believe that the term Belfield will retain its utility if it is redefined as the clastics between the Minnekabta Limestone and the unconformity. It is recognized by higher resistivity readings on the laterolog. The overall distribution of tbe redefined Belfield Mem­ber with it' zero edge closely parallel to the 35-foot contour of the Minnekabta Limestone and the rapid thinning from 50 feet to a zero edge to the north and east indicate erosional limits for the Belfield Member in those areas (fig. 6).

The Opeche and Minnekabta have always been assigned a Permian age. The Pine Salt bas been correlated as Permian (Rascoe, 1972, Bluemle, et aI., 1981) which then requires that the Belfield Member, as redefined, be assigned a Permian age.

Permo-Triassic

The term Pine would have its greatest utility if it were redefined similar to its original definition to include halites and associated clastics. Deposition would then begin with the clastics between the Belfield Member and the "P Salt" continuing through the "G Salt" and concluding with clastics above the "G Salt" (see cross sections A-A' and B­B', plate 1).

5

The "G Salt" (fig. 7) appears to have depositional thicknesses preserved. The "P Salt" (fig. 8) also has depositional thicknesses preserved in most areas. The eastern margin of the "P Salt" is likely a solution margin and there are some areas, particularly along the Nesson Anticline, where solution may have altered thicknesses. The preservation of depositional thicknesses of these halites in most areas indicates that clastic deposition must have succeeded evaporite deposition before any eFosional episode.

The red clastics between the Belfield Member and the "P Salt" are approximately the same thickness as the Pine clastics to the east of the limit of the "P Salt" in southwestern North Dakota. If we accept the Permian age ascribed to the Pine Salt (Rascoe, 1972), then these clastics must also be of Permian age.

The area of greatest thickness of the Pine Member (fig. 9) is in the area where the "G Salt" is present and in the area of maximum thickness of the "P Salt." In those areas, clastics are relatively thin in the upper portion of the Pine Member. Proceeding northward, as the "P Salt" thins, the clastics below the "P Salt" thin while the clastics overlying the "P Salt" thicken (plate I). Assuming a facies relationship between the evaporites and the clastics, this would imply a Permian age for most of the Pine clastics where halite is absent.

The "G Salt" has been correlated with the Little Medicine Tongue ofthe Goose Egg Formation of Wyoming which is considered to be Triassic in age. This correlation would imply a Triassic age for the overlying clastics and a Permo-Triassic age assignment for the redefined Pine Member (fig. 2).

Jurassic

The Rierdon and Piper Formations are ofun­questioned Jurassic age and have depositional thick­nesses preserved in most areas of western North Dakota. The Piper Formation, as currently recognized in North Dakota, is equivalent to the Nesson Formation as defined by Nordquist (1955) plus the Dunham Salt. Nordquist (1955) defined six members and these members are recognizable in the central basin area, but facies changes make it difficult to trace the units south and eastward. Current Montana usage is to restrict the term

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Figure 8. Isopach map of "P" Salt.

11

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Area of "G" & 'p' Salt 1::::~J] Area of 'p' Salt ~

Figure 9. Isopach map of Pine Member of the Spearfish Formation,

12

Nesson Formation 10 the Kline, Picard and Poe Members with the upper members transferred10 the Piper Formation. The lower Piper, as currently defined in North Dakota, is a redbed and evaporite depositional episode which includes the Picard Shale, Poe Evaporite, and Dunham Salt. The upper Piper consists of a normal marine depositional sequence of shales and IimeslOnes of the Bowes, Firemoon, Tampico, and Kline Members. The top of the Rierdon Shale and the top of the Picard Shale provide two consistent picks to provide the normal marine Jurassic regional depositional pattern. It shows a gradual northwest to southeast thinning of these strata from nearly 500 feet in northwestern North Dakota to less than 200 feet at the South Dakota border (fig. 10).

Zieglar introduced the term Saude to refer to clastics between the Dunham and Pine Salts assuming a Jurassic age for both salts. Subsequent interpretations have agreed with the Jurassic age of the Dunham Salt, but the clastics have generally been regarded as Triassic in age in North Dakota. The term Saude is derived from the Phillips Petroleum Company - OliviaSaudeNo. I well (NW SE 19-158-74) where salts are absent and redbeds lie unconformably on the Tilston Interval of the Madison Formation (Mississippian). Presumably the name was chosen because of the cored interval in that well (fig. 11) although Zieglar did not desig­nate this as the type section. Zieglar used a composite log from an area of the basin where all the salts are present to define the Saude (fig. 1). Dow used a well in the same area to redefine the Saude and then designated the interval between 6290 feet and 6610. feet in the Amerada Petroleum Corporation - Pederson, Cater No. I well (NE SW 21-158-95) as a type section (fig. 12). Presumably he chose that well because he wanted to redefine the Saude as the interval between the Poe Evaporite and the "P" rather than the "G" salt (fig. I). As thus defined, the Saude was considered to be Triassic since the top of the Permian was placed at the top of the "P" (Pine) Salt. Dow described the upper 60 feet, based on cuttings, as moderate reddish brown, slightly calcareous, silty shale. The remainder of the Saude was described as moderate reddish orange siltstone, sandy in part, with rounded, frosted sand grains. Dow (1967, p. 19) thought it probable that the Jurassic-Triassic boundary was in the upper part of the Saude. Zieglar placed the Jurassic uncon­formity at the base of the Pine Salt so he considered

13

the Saude 10 be Jurassic in age.

I would redefine the Saude Member of the Piper Formation as the interval from 2802 10 2848 feet in the Phillips Petroleum Company - O. Saude No. I well (fig. 13). This interval is equivalent to an upper portion ofDow's type section, the interval from 6290 to 6405 feet in the Amerada Petroleum Corporation - Pederson, Cater No.1 well (fig. 12). As thus defined, the Saude is generally less than 100 feet thick, but extends south and east of the limit of the Pine Member of the Spearfish Formation and represents the initial Jurassic deposition in the Williston Basin (fig. 13).

The Dunham Salt (fig. 14) i;limited to the central basin area and has an erratic distribution due 10 dissolution in many areas. One ofthese areas has been demonstrated as Post-Piper dissolution (Dow, 1967, pI. 11), but no attempt has been made to document time ofdissolution in other areas. Where depositional thicknesses of the Dunham Salt are present the Dunham Salt thickens with compensated thinning of Poe anhydrites (plate 2) This demon­strates the facies relationship of the evaporites and accounts for the Jurassic age assignment for this salt.

Unconformities

The subtle lithologic changes within the redbeds make recognition of unconformities difficult, but log characteristics on Dual Laterologs (plates 1-2) are consistent with regional uncon­formities within the redbeds. Recognition of the unconformity between the Belfield and Pine Members of the Spearfish Formation is aided by the areal extent of the higher resistivity redbeds to the areal extent of the underlying Minnekahta Lime­stone. Recognition ofthe unconformity between the Saude and Pine redbeds is more tenuous, but utilizes a combination of the gamma ray and resistivity characteristics. There is also generally a higher resistivity below this unconformity and the gamma ray characteristics are consisteut with the generally noted more shaly lithology of strata immediately below the Poe (or Amaranth) Evaporites. Place­ment of pre- "P" (Pine) and pre-Dunham unconformities witbin the red clastics provides a more logical depositional sequence than initial thick salt deposition.

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Figure 10. Isopach map of Reirdon Formation and Bowes through Kline Members of the Piper Formation.

14

PHILLtPS-o. SAUDI MO. 1

SEC. ll,T. 158 N.,.R. 74 w. BO'nDfEAU COURTY'. NORTH DAKOI'.I..

....

E

Figure 11. Log of Phillips Petroleum COEp. - O. Saude No. I well (NW SE 19-158-74) with revised definition of Saude Member of Piper Formation.

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Figure 12. Log ofAmerada Petroleum Corp. - Pederson, Cater No.1 well (NE SW 21-158-95) showing revised definition of Saude Member of Piper Formation.

15

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Figure 13. Isopach map of Saude Member, Piper Formation.

16

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F"Jgure 14. Isopach map of Dunham Salt, Piper Formation.

17

The unconformity between the Opeche Formation or older Paleozoic strata and overlying redbeds has been referred to as a pre-Mesozoic event (Anderson, 1974). The nature of the contact in southeastern Saskatchewan, southwestern Manitoba, and north-eentral North Dakota is one of essentially unweathered Mississippian carbonates with only minor secondary anhydrite in areas where the overlying redbeds are referred to as the Pine Member. This is in sharp contrast to south-central North Dakota where a karst topography has developed to a depth of 130 feet in the Herman Hanson - Welder No. I well (NE NW 20-133-72; Carlson, 1958) where red shales of the Saude Memberofthe Piper Formation overlie Mississippi­an carbonates (fig. 15). This indicates a significant age difference in different areas for the uncon­formity between redbeds and Paleozoic strata in North Dakota. I would SUbmit that the elusive pre­Middle Jurassic unconformity is at the horizon which I have designated as the pre-Saude uncon­formity.

Global sea level lowstands of Early Pennsylvanian, Mid-LeonardianandSinemurianage with associated unconformities fit nicely with unconformities at the base of the Tyler, Opeche, and Piper (Saude Member) Formations. The unconformity at the base of the Pine, if associated with the relative sea level fall al the base of the Middle Triassic would indicate a Triassic age for the Pine Member. Preservation of depositional thicknesses of the Pine halites below the unconformity associated with the Early Jurassic sea level lowstand would seem more likely if the Pine were deposited during the Triassic relative sea level rise than during the Permian. However, if the Wyoming correlations are correct, the pre-Pine unconformity is a mid-Guadalupian (Permian) event.

CONCLUSIONS

The Spearfish Formation, as currently defined in North Dakota, is generally consistent with type Spearfish, i.e., it contains strata of probable Permian to Jurassic age. However, common usage, and quite likely the original intent was to restrict Spearfish to the pre-Jurassic strata. A formation should be a lithogenetic unit; as such the Belfield and Pine might be referred to as formations, but they are equivalent to the Permo-

NDF NO. 1835 H. HANSON.WELDER NO.f NENW SEC. 20, T.133N., R.72W.

S.P. RES.

~ z 0 !!

o~ z -==: Q

~-~ o~No; O!! ~

~

..... • <~

.... !! "'"­<s... ~

Figure 15. Log characteristics of Jurassic and Mississippian strata in the Herman Hanson - Welder No. I well (NE NW 20-133-72) in Logan County.

Triassic portion of the type Spearfish so I would recommend that they be regarded as members of the Spearfish Formation.

A Permian age assignment for the Belfield Member is consistent with the conformable relalionship to the Minnekahta Limestone and the age assignment of the Minnekahta in other areas. The age of the Pine Member is less clear. The "G" salt has been correlated with the Little Medicine Tongue of the Goose Egg Formation of Wyoming

18

which is regarded as near the base nf the Triassic. Thatinterpretation indicatesa primarily Permianage for the Pine Member. Physical evidence in the Williston Basin seems more consistent with a Triassic age assignment, but the correlations to strata regarded as Permian in Wyoming is also plausible. Therefore a Permo-Triassic age assign­ment for the Pine seems appropriate until more definite age criteria are established.

A pre-Middle Jurassic unconformity has heen generally recognized, but the physical placement has heen controversial. Placement ofthis

unconformity at the base of the redefined Saude is consistent with Canadian observations of a grada­tional or conformable contact between the redbeds and the overlying Jurassic evaporites and carbon­ates. Limiting the pre-Middle Jurassic on Paleozoic strata unconformity to the area beyond the limits of the Pine Member of the Spearfish Formation is consistent with the differing weathering characteris­tics of the Paleozoic carbonates and evaporites in those areas. Placing the pre-Middle Jurassic unconformity within the redbeds in the subsurface is consistent with the relationship in the outcrops in the Black Hills.

19

REFERENCFS

Andenon, Sidney B., 1974, Pre-Mesozoic Paleogeologic Map of North Dakota: North Dakota Geological Survey Miscella­neous Map 17.

Barchyn, D., 1982, Geology and Hydrocarbon Potential of the Lower Amaranth Forma­tion, Waskada-Pienon Area, Southwestern Manitoba: Manitoba Department of Energy and Mines Geological Report, OR 82-6, 30 p.

Bluemle, J.P., et al., 1981, Williston Basin Stratigraphic Nomenclature Chart: North Dakota Geological Survey Miscellaneous Series 61.

Bluemle, J.P., et aI., 1986, North Dakota Strati­graphic Column: North Dakota Geological Survey Miscellaneous Series 66, 3 sheets.

Carlson, C. G., 1968, Triassic-Jurassic of Alberta, Saskatchewan, Manitoba, Montana and North Dakota: American Association of Petroleum Geologists Bulletin, v. 52, p. 1969-1983.

Carlson, C. G., 1958, Summary of the Herman Hanson Oil Syndicate-Barbara, Ann and Theresa Welder No.1 well: North Dakota Geological Survey Circular 211, 8 p.

Christopher, J. E., 1982, Depositional Patterns and Oil Field Trends in the Lower Mesozoic of the Northern Williston Basin, Canada: in Oil and Gas of Saskatchewan, Saskatche­wan Geological Society Special Publication No.7, p. 83-102.

Cummings, A. D., 1956, The Watrous Strata in Saskatchewan, in Fint Williston Basin Symposium: North Dakota and Saskatche­wan Geological Societies, p. 165-169.

Darton, N. H., 1899, Jurassic Formations of the Black Hills of South Dakota: Bulletin Geological Society of America, v. 10, p. 383-396.

Dow, W. G., 1%7, The Spearfish Formation in the

Williston Basin of Western North Dakota: North Dakota Geological Survey Bulletin 52, 28 p., 17 pis.

Francis, D. R., 1957, Jurassic Stratigraphy of the Williston Basin Area: American Association Petroleum Geologists Bulletin, v. 41, p. 367­398.

Goldsmith, J. W., 1959, in McKee, and othen, Paleotectonic maps of the Triassic System: U. S. Geological Survey, Miscellaneous Geological Investigation Map 1-300, p. 4, II.

Husain, Muzaffar, 1991, Regional geology and petroleum potential of tbe Lower Amaranth Formation, Coulter-Pierson area, southwest­ern Manitoba: in Sixth Williston Basin Symposium, Saskatchewan, Montana and North Dakota Geological Societies, p. 151­160.

Imlay, R. W., 1947, Marine Jurassic of the Black Hills Area, South Dakota and Wyoming: American Association Petroleum Geologists Bulletin, v. 31, p. 227-273.

Kreis, L. K., 1991, Depositional History of the JurassicSystem and Hydrocarbon Accumula­tions in the Wapella-Moosomin area, South­eastern Saskatchewan: in 6th Williston Basin Symposium Saskatchewan, Montana and North Dakota Geological Societies, p. 165­178.

Laird, W. M., 1951, Discovery heightens interest in North Dakota geology: World Oil, June­July 1951, 7 p.; North Dakota Geological Survey Report oflnvestigations 3.

LeFever, J. A., Martiniuk, C. D., and Anderson, S. B., 1991, Correlation cross-sections along theUnited States-Canadalntemational border [North Dakota-Manitoba]: North Dakota Geological Survey Report of Investigations 92, 5 sheets.

McLachlin, M. E., 1972, Triassic System: in Geologic Atlas of the Rocky Mountain Region; edited by Mallory, W. W., Rocky

20